Fiber reinforced plastics (FRP) comprising reinforcing fibers such as carbon fibers, glass fibers or aramid fibers are used as material for structural members of automobiles and aircraft, etc. because they are light in weight and high in durability.
An autoclave molding has been known as a method to mold fiber reinforced plastics (FRP). With this molding method, a layered product of prepreg sheets comprising reinforcing fibers and high-ductility epoxy resin, for instance, is pressed and heated in an autoclave for curing to produce fiber reinforced plastics (FRP).
It is generally difficult, however, for prepreg sheets to serve for molding products having a complicated three-dimensional shape. Under the existing circumstances, autoclave molding of conventional prepreg sheets has not been widely practiced because it needs high material costs and long molding process times, which will lead to high overall product manufacturing costs.
Compared to this, a resin transfer molding (RTM) and a vacuum RTM are attracting attention because these molding methods can work at lower costs and shorter molding process times than an autoclave molding of conventional prepreg sheets.
In the RTM process, a layered product of dry reinforcing fiber cloths having no impregnated matrix resin is placed in a mold and a low-viscosity liquid matrix resin is injected to allow the reinforcing fibers to be impregnated with the matrix resin to achieve molding of fiber reinforced plastics (FRP).
As the RTM process uses the dry reinforcing fiber cloth as described above, it is possible to shape the reinforcing fiber cloth along a complicated three-dimensional shape in a mold. However, wrinkle-free, uniform fiber reinforced plastic (FRP) products having a high volume percentage of fiber (Vpf) such has those produced by an autoclave molding of prepreg sheets cannot be produced easily by simply placing the layered product of reinforcing fiber cloths along a surface of the mold. The volume percentage of fiber is defined as the percentage by volume of fiber relative to the total volume of the material containing that fiber.
In a method to solve this problem, there is a method using a preform that is pre-formed in a shape of a final product in a dry state without a matrix resin impregnation. However, the time required to prepare such a preform and the accuracy of the resulting preform will have large influence on the production cost and quality of the final fiber reinforced plastic (FRP) product.
Thus, a preform production process that can be performed in a shorter time is proposed in Patent Literature 1.
The process disclosed in Patent Literature 1, however, cannot work satisfactorily to provide a wrinkle-free, uniform preform having a high volume percentage of fiber (Vpf) that can be used for molding of fiber reinforced plastics (FRP) having high mechanical characteristics such as those for aircraft's structural members.